For long span double-deck suspension bridges with steel truss girder, a new aeroelastic model design method is proposed, referred to as the multi-steel beam frame system (MSBFS), which comprises upper and lower steel beams, horizontal connecting rigid beams, and vertical rigid columns. By transforming the design process into an optimization problem and employing the constrained pattern search method, the design variables for the stiffening beam satisfying the design criteria are obtained. The sensitivity analysis reveals that the modal frequencies are significantly influenced by the dimensions of the horizontal connecting rigid beams, as well as the upper and lower longitudinal steel beams. The dynamic characteristic test results confirm the accuracy of the MSBFS, as the deviation in low-order modal frequencies from the design target values falls within ±5%. As a valuable complement to the full-bridge aeroelastic model design approaches, this design method is well-suited for truss girders, enabling an overall optimal simulation of aerodynamic shape, stiffness and mass system.